The polar vortex is really just a large air mass that is extremely cold (temperatures fall below -78C, or -108F, during the Northern Hemisphere winter). This concentrated area of cold is encircled by a fast-flowing river of air called the polar night jet. Basically, the jet – with its swiftly moving air current – traps the vortex over and near the poles, north and south.
Interactive: What is a polar vortex, and why is it here?
Though the vortex is present throughout the year, it really comes alive toward the winter. Temperatures cool off dramatically with the loss of sunlight in the polar region. The cooling temperatures force the vertical layer of air within the vortex to shrink because the colder air becomes, the more dense it gets. The increased density lowers the volume of air (density and volume have an inverse relationship), hence the shrinkage.
On a weather map, meteorologists identify areas of low and high “heights” to identify the polar vortex. The low heights represent regions where the vertical layer of air at a certain elevation is compressed, while the high heights indicate regions where the layer of air is expanded. Think of it this way: an area of low heights signifies that the air here is quite dense and, hence, cold; an area of high heights tells you that the air here is quite light and, hence, warm.
The polar vortex is most certainly an area of low heights. Meteorologists most commonly analyze it at the 500 mb pressure level, which is roughly 18,000 feet above the ground. At this level, it looks like a massive low pressure system with a characteristic counterclockwise circulation.
There are times during the winter when this circulation goes off the rails. Waves continually move upward through the atmosphere in the Northern Hemisphere; sometimes, these waves crash into the polar vortex. If this wave-crashing activity is strong and persistent enough, the vortex will begin to weaken and parts of it – literally chunks of brutally cold air – will break off into one or all of the three big continents (Europe, Asia and North America).
Below is a loop of the 500 mb heights over the last 30 days. Blue colors indicate areas of relatively low heights. At the beginning of the loop in early-mid December, the polar vortex is easily identifiable. Note how it had a “home base” between Canada and Greenland (close to Baffin Bay). In late December, the vortex began to split apart and, by the start of the new year, a large piece of it escaped to the south over Quebec. Bitterly cold air accompanied this piece of the vortex last Friday and Saturday, with snow-covered areas of the Northeast feeling the most extreme cold.
Skip ahead a few days and note that, while the Friday-Saturday vortex chunk has made it across the Pond, a new and stronger piece of the vortex has dug deep into the U.S. (see the 500 mb height analysis from Monday afternoon on the first image below; again, blue colors depict areas of relatively low heights and cold temperatures). This particular version of the vortex was able to make it farther south thanks to a well-timed combination of blocking features to the west, north and east. The orange and red colors are the aforementioned areas of high heights, under which relatively warm air exists. Under this scenario, polar air can only go in one direction: southward (see the spillage of this intense cold into the northern reaches of the U.S. on the second image below).
It’s one thing when a “vortex split” occurs, but quite another when a detached piece lands in the Northern U.S. Some of the more historic cold outbreaks of the past 30 years (along with many that predate the early 1980s) have featured a major displacement of the vortex into the Lower 48. Six such examples appear below.
Each individual vortex split event will have its own effects on the U.S., with some rather benign and others producing frigid temperatures across a wide swath of the country. A large expanse and deep layer of snow cover is helping to maximize the cold potential of the current event. Snow is a highly efficient reflector of sunlight and, as such, slows the rate at which the Sun heats the surface and warms air temperatures.
In sum, the polar vortex is not some mysterious or cataclysmic feature. We would consider its existence unremarkable most years, but now, because a piece of it has split off and slid into the U.S., the vortex is stealing headlines for the winter season.